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"All natural" -- it's on the labels of a growing number of foods, cosmetics, cleaning products, and over-the-counter remedies. This is, in part, what makes herbal medicine so popular. But does natural always mean safe?

Herbal medicine is the use of plants as medicine. Typically taken by mouth or applied to the skin, medicinal herbs can come in several forms, such as ointments, oils, capsules, tablets, and teas.

Though many people may use them as medicine, herbal supplements are not regulated by the FDA like prescription and over-the-counter drugs are. For this reason, some potentially dangerous herbs may be available in stores, online, and even in local coffee shops. You take them at your own risk. Before taking any herb, be sure to research it and talk to your health care providers -- doctors, pharmacists, and anyone else who's involved in your medical care.

Risky Herbs

"Some people think herbal supplements really work but that they are harmless," but if it acts like a drug in the body, then it can also have a negative effect, says Adriane Fugh-Berman, MD, an expert on medicinal herbs and dietary supplements and a professor at Georgetown University School of Medicine.

"Most herbs we use in the U.S. are pretty benign," Fugh-Berman says, "but some are dangerous and others are if not taken correctly."

Anything that works like a drug is going to have some risks, says Cydney McQueen, PharmD, a professor at the University of Missouri-Kansas City School of Pharmacy.

For herbs that pose major risks, the most common risks are liver and kidney damage and drug interactions.

Here are examples of herbs that carry risks you may not know about. This is not a complete list of every potentially risky herb or other supplement; it simply shows that some very risky substances are available to anyone over the counter. So again, be sure to talk to your health care providers before taking any herbs.

St. John's Wort

St. John's wort (Hypericum perforatum) can ease mild to moderate depression, says Andrew Weil, MD, who is the founder and director of the Arizona Center for Integrative Medicine at the University of Arizona. But there is not enough evidence that it helps with major depression.

Besides, depression isn't something to treat without help. "It's not the common cold. If someone wants to use St. John's wort for depression, they still must be managed by a health care provider," McQueen tells WebMD.

Here's one major reason why: drug interactions. St. John's wort can make many other drugs less effective. There have been cases of unintended pregnancies in women taking St. John's wort and birth control pills and cases of organ rejection in those taking St John's wort with anti-rejection drugs after a transplant.

"If you are taking any prescription drug and are interested in trying a course of St. John's wort for mild to moderate depression, first discuss possible interactions with your doctor or pharmacist," says Weil, whose line of dietary supplements includes a product containing St. John's wort.

Kava

Kava (Piper methysticum) can reduce anxiety, and for some it has worked as well as prescription anti-anxiety drugs. But it may take up to eight weeks to work. In women experiencing anxiety in menopause, kava has worked in as little as one week, according to the National Institutes of Health.

However, the National Institutes of Health and the FDA urge people not to take kava because of the risk of serious illness, liver damage, and death even when taken for only a short time at normal doses. Kava use has led to liver transplants and death in one to three months. "Heavy kava use has been linked to nerve damage and skin changes," Weil tells WebMD.

Kava can worsen depression and is not safe for women who are pregnant or breastfeeding. Because the herb has effects similar to those of alcohol, the two should not be combined.

A number of prescription drugs should not be combined with kava. The two drugs with the potential for greatest drug interactions are alprazolam (Xanax) and sedatives.

Weil only recommends kava for a maximum of three to four weeks in patients with healthy livers. "I do not recommend kava for people at risk for or who have liver disease, regularly drink alcohol, or take drugs with known adverse effects on the liver, including statins and acetaminophen."

Other experts have completely ruled kava out. "I prefer to use herbs that have a good risk-to-benefit ratio, and for kava that's no longer true," Fugh-Berman says.

Comfrey

Comfrey (Symphytum officinale) has "a well-deserved reputation for healing injured tissues," such as wounds, bruises, sprains, bone fractures, and the swelling and inflammation that can go along with them, Weil tells WebMD. But because of the risk for severe liver and possibly lung damage, "comfrey should never be taken by mouth," Weil says.

The FDA recommended in 2001 that manufacturers remove comfrey products from the market. Still, comfrey is easy to find.

"My local coffee shop serves comfrey tea, and when I told them it was a liver-toxic herb, they said, ‘Oh, we sell a lot of it," Fugh-Berman says.

Weil recommends applying comfrey to wounds that don't heal easily, including open bedsores and diabetic ulcers. However, the U.S. Pharmacopeia, a scientific organization that sets standards for dietary supplements, advises against using comfrey on broken skin, as the toxins that may affect the liver can be absorbed.

Chaparral

Chaparral (Larrea divaricata) is said to reduce pain, inflammation, and skin irritation. However, there is little evidence for this, Weil tells WebMD. Chaparral has also been promoted as a cancer-fighting herb, but according to the American Cancer Society, there is no evidence supporting that, either.

Easily found online in many forms, chaparral has been listed in the FDA's poisonous plant database since 1997 because of the risk of severe -- and in some cases, irreversible -- liver damage.

According to the American Cancer Society, chaparral can cause serious drug interactions with some prescription and over-the-counter drugs, including blood thinners; anti-inflammatory drugs, such as aspirin, ibuprofen, and naproxen; diabetes medications, and certain antidepressants.

Pennyroyal

Pennyroyal (Mentha pulegium) has not been proven effective for any suggested uses. It was traditionally used to cause abortion, but the large doses required for this could kill the mother or cause irreversible damage to the liver and kidneys, according to the National Institutes of Health.

According to the National Institutes of Health, pennyroyal oil is considered unsafe for anyone at any dose, and it is unknown whether the tea is safe.

"It's a mint, and you don't get that much poison in a tea, but I wouldn't risk it. Go for spearmint. Why go for the liver-toxic mint?" Fugh-Berman says.

Listed in the FDA's poisonous plant database in 1997, pennyroyal can be found online in many forms, including oil.

How Can I Choose a Safe Herbal Supplement?

These are steps everyone should take before taking any herb, says Tod Cooperman, MD, who is president of ConsumerLab, which tests the safety and quality of dietary supplements.

Do your homework. Before starting any herbal medicine, find out:

Is it safe?

Does it work?

What dosage works?

What part of the plant works? (root, stem, leaf)

Talk to your health care team. Tell all of those involved in your health care -- physical and mental health -- that you're considering taking an herbal supplement. Discuss whether the supplement is safe and effective in general and for you specifically. Remind health care providers of any conditions you have and any prescription or over-the-counter medications you take. Don't wait for them to ask.

Get a quality product. Check the label for the plant's common and Latin names and the plant part used, Weil advises. If it's the root that's effective, you won't benefit from tablets made from the stem.

Look for a quality seal. "Herbals are the most likely of all supplements to contain contaminants," Cooperman says. The three major quality seals are the USP seal (US Pharmacopeia), the NSF seal (National Sanitation Foundation), and the CL seal, issued by Cooperman's Consumer Lab.

Each of these seals indicates that the product ingredients match the label and that if there are contaminants present, they do not exceed safe levels. USP and NSF make sure the product meets Good Manufacturing Practices set by the FDA. CL holds products to standards set by the state of California, which are more stringent than the FDA's standards, Cooperman says. USP and CL also verify that supplements will break apart in the body.

Go for supplements made by big companies, McQueen suggests. Major store brands or manufacturers of FDA-regulated drugs are the most likely to adhere to quality standards.

Test tablets. "Typically herbals are powders in capsules that you don't need to worry about, but make sure tablets will break apart and release ingredients in your body," Cooperman says. Put the pill in body-temperature water and give it about 45 minutes to fall apart. "If it stays intact, it's likely doing the same thing in your body," Cooperman says.